Into a recordable or rewritable recording medium such as a DVD-R (DVD-Recordable) and a DVD-RW (DVD-Re-writable), data is recorded by irradiating a recording surface of the optical disc with a laser beam. In the portion irradiated with the laser beam on the recording surface of the recording medium, a condition of a recording film is changed due to an increase in temperature, which forms a record mark on the recording surface.
Thus, by modulating the laser beam with a recording pulse having a time width corresponding to the data to be recorded and thereby generating a laser pulse with a length corresponding to the data to be recorded, and by applying the generated laser pulse to the recording medium, it is possible to form the record mark with a length corresponding to the data to be recorded, on the recording medium.
On the other hand, in order to improve an SN ratio of a read signal read from the recording medium on which the data is recorded at high density, there is known a technology in which a filtering process for emphasizing high frequencies is performed on the read signal, for waveform equalization. In particular, according to a patent document 1, the technology is disclosed that the high frequencies can be emphasized without any intersymbol interference by performing the filtering process after amplitude limit is performed on the read signal (a technology about a so-called limit equalizer).    Patent document 1: Japanese Patent No. 3459563